As integrated circuits have increased in integration and evolved into multilayer wiring in recent years, the semiconductor wafers on which these integrated circuits are formed need to be flat to a high degree of precision.
Chemical-mechanical polishing (CMP) is a known polishing method for polishing semiconductor wafers. CMP involves polishing the surface of a substrate to be polished with a polishing pad while a slurry of abrasive grains is dropped onto the surface.
The following Patent Documents 1 to 4 disclose polishing pads used in CMP, which are composed of a polymer foam having an independent cell structure and being manufactured by foaming a two-part curing type of polyurethane. These polishing pads are stiffer than the nonwoven cloth type of polishing pads described below, and therefore can be used to advantage in the polishing of semiconductor waters, where a high degree of flatness is required.
A polishing pad composed of a polymer foam having an independent cell structure is manufactured, for example, by subjecting a two-part curing type of polyurethane to a cast-foam-molding. Because such a polishing pad has relatively high stiffness, the load tends to be applied during polishing selectively to the convex parts of the substrate being polished, and as a result, the polishing rate is relatively high. However, if agglomerated abrasive grains are present on the polishing surface, the load will also be selectively applied to these agglomerated grains, making the polishing surface more susceptible to scratching. In particular, as described in Non-Patent Document 1, when a substrate having copper wiring that is easily scratched, or a material with a low dielectric constant and low interfacial adhesive strength is polished, scratching or interfacial separation is especially apt to occur. Also, in a cast-foam-molding, it is difficult to foam a macromolecular elastomer uniformly, which means that there tends to cause variance in the flatness of the substrate being polished and in the polishing rate during polishing. Furthermore, with a polishing pad having independent pores, the voids originating in the independent pores can become clogged with abrasive grit and polishing dust. As a result, when the pad is used for an extended period, the polishing rate decreases as polishing proceeds (this feature is also referred to as polishing stability).
Meanwhile, as an another type of a polishing pad, Patent Documents 5 to 14 disclose a nonwoven cloth type of polishing pad, which is obtained by impregnating a polyurethane resin into a nonwoven cloth and by subjecting to wet solidification. These nonwoven cloth polishing pads have superior flexibility, and therefore when agglomerated abrasive grains are present on the polishing surface of the substrate being polished, deformation of the polishing pad will reduce the selective application of load to the agglomerated abrasive grains. However, because the nonwoven cloth polishing pads are flexible, the polishing rate is low. Also, because the polishing pad deforms by conforming to the surface shape of the substrate being polished, high flattening performance (the characteristic of making the polished substrate flat) is not obtained.
In recent years there has come to be known a nonwoven cloth polishing pad that is obtained using a nonwoven cloth formed from ultrafine fiber bundles, the goal of which is to obtain better flattening performance (see the following Patent Documents 15 to 18). More specifically, Patent Document 15, for example, discloses a polishing pad formed in the form of a sheet comprising a nonwoven cloth produced by entangling polyester ultrafine fiber bundles with an average size of 0.0001 to 0.01 dtex, and a macromolecular elastomer whose main component is polyurethane and which is present in the spaces inside this nonwoven cloth. It is stated that this polishing pad affords more precise polishing than in the past.
However, since the polishing pads disclosed in Patent Documents 15 to 18 make use of a nonwoven cloth obtained by needle punching ultrafine staple fibers that are small in size, the apparent density is low and the percentage of void is high. Accordingly, only a polishing pad that is flexible and low in stiffness can be obtained, which means that the pad deforms while conforming to the surface shape, so sufficiently high flattening performance is not obtained.                Patent Document 1: Japanese Patent Application Laid-Open No. 2000-178374        Patent Document 2: Japanese Patent Application Laid-Open No. 2000-248034        Patent Document 3: Japanese Patent Application Laid-Open No. 2001-89548        Patent Document 4: Japanese Patent Application Laid-Open No. H11-322878        Patent Document 5: Japanese Patent Application Laid-Open No. 2002-9026        Patent Document 6: Japanese Patent Application Laid-Open No. H11-99479        Patent Document 7: Japanese Patent Application Laid-Open No. 2005-212055        Patent Document 8: Japanese Patent Application Laid-Open No. H3-234475        Patent Document 9: Japanese Patent Application Laid-Open No. H10-128674        Patent Document 10: Japanese Patent Application Laid-Open No. 2004-311731        Patent Document 11: Japanese Patent Application Laid-Open No. H10-225864        Patent Document 12: Japanese Translation of PCT Application No. 2005-518286        Patent Document 13: Japanese Patent Application Laid-Open No. 2003-201676        Patent Document 14: Japanese Patent Application Laid-Open No. 2005-334997        Patent Document 15: Japanese Patent Application Laid-Open No. 2007-54910        Patent Document 16: Japanese Patent Application Laid-Open No. 2003-170347        Patent Document 17: Japanese Patent Application Laid-Open No. 2004-130395        Patent Document 18: Japanese Patent Application Laid-Open No. 2002-172555        Non-Patent Document 1: Masahiro Kashiwagi et al., “Science of CMP,” Science Forum, Inc., Aug. 20, 1997, pp. 113-119        